Electrode‑assisted trapping and of droplets on hydrophilic in a hydrophobic microchannel

In two-phase flow microfluidics, there is an increasing interest in technologies which enable the encapsulation of biological cells into aqueous drops and the subsequent study of their molecular (excretion or lysis) products. One not yet available but very promising analysis method is the use of biospecific surface patches embedded in the wall of microfluidic channels. In this paper, we tackle some technological challenges encountered in the development of such applications. In the detection protocol, each drop must be enabled to wet the designated patch, be held in contact long enough for biomolecular detection and subsequently be released. This is engineered via a combination of well-defined chemical sites in the walls of the flow channel and insulated microelectrodes. The tunability of the local electric field allows to modify the competition between chemical (pinning) forces which tend to immobilize the drop and hydrodynamic forces which oppose this process. We developed a prototype microfluidic device which offers this functionality. A channel structure is sandwiched between an actuation surface with electrowetting (EW) electrodes on one side and a detector surface with a hydrophilic patch amidst a hydrophobic environment on the other. Two pairs of carefully aligned EW electrodes are used: one for drop adherence and another one for the subsequent release. We demonstrate these operations and discuss the required voltage signals in terms of the forces on the drop. Finally, we discuss possible steps for further improvement in the device

High-throughput sorting of drops in microfluidic chips using electric capacitance

A theoretical model has been developed to analyse bubble rise in water and subsequent impact and bounce against a horizontal glass plate. The multiscale nature of the problem, where the bubble size is on the millimetre range and the film drainage process happens on the micrometre to nanometre scale requires the combined use of different modelling techniques. On the macro scale we solve the full Navier–Stokes equations in cylindrical coordinates to model bubble rise whereas modelling film drainage on the micro scale is based on lubrication theory because the film Reynolds number becomes much smaller than unity. Quantitative predictions of this model are compared with experimental data obtained using synchronised high-speed cameras. Video recording of bubble rise and bounce trajectories are combined with interferometry data to deduce the position and time-dependent thickness of the thin water film trapped between the deformed bubble and the glass plate. Bubble rise velocity indicated that the boundary condition at the bubble surface was tangentially immobile. Quantitative comparisons are presented for bubbles of different size to quantify similarities and differences